Flame monitoring system

ABSTRACT

Disclosed is a burner control system in which fuel flow is maintained by an operator in response to a flame signal produced by a capacitor charged by flame rectified current. System operation is initiated by precharging the capacitor from an auxiliary direct current source while preventing the operator from supplying fuel. Subsequently, the operator is activated simultaneously with disconnection of the auxiliary direct current source from the capacitor.

United States Patent t191 Walbridfge [45] July9, 1974 [54] FLAMEMONITORING SYSTEM Primary Examiner-Edward G. Favors [75] Inventor' H'Walbndge Ashland Attorney, Agent, or Firm-John E. Toupal [73] Assignee:Walter Kidde & Company, Inc.,

Clifton, NJ.

[22] Filed; June 23, 1972 [57] ABSIRACT 21] Appl. No.: 265,598 Disclosedis a burner control system in which fuel flow Related U.S. ApplicationData Division of Ser. No. 220,788, Jan. 26, 1972.

U,.S. Cl. 431/78 Int. Cl. F23n 5/12 Field of Search 431/78, 80, 264, 254

is maintained by an operator in response to a flame signal produced by acapacitor charged by flame rectied current. System operation isinitiated by precharging the capacitor from an auxiliary direct currentsource while preventing the operator from supplying fuel. Subsequently,the operator is activated simultaneously with disconnection of theauxiliary direct current source from the capacitor.

13 Claims, 5 Drawing Figures l ruim MONITORING SYSTEM BACKGROUND OF THEINVENTION This invention relates generally to fuel burners and, moreparticularly, to fuel control systems for fuel burners.

Extensive efforts have been directed toward the irnprovement of fuelcontrol systems for fuel burners such as gas and oil burners and thelike. Increased system safety and reliability have been primaryobjectives of such efforts. These objectives, however, generallyconflict with an obvious desire to limit the cost and physical size ofthe systems.

Most burner systems employ fuel supply valves that are automaticallycontrolled by some type of flame sensing mechanism that automaticallyinterrupts fuel flow in response to a predetermined loss of flamecondition. According to one common technique, the presence of a flame isindicated by a signal current which is rectified by the flame as aresult of the well known ionization phenomena. Although flamerectification provides a relatively effective method of sensing flame,prior systems of this type have suffered from certain disadvantagesincluding the requirement for expensive isolation transformers forisolating the flame sensing circuitry from the power lines. Otherproblems of prior systems are associated with the necessity forisolating the d.c. flame rectification signal from a.c. componentpresent therewith. In many poor flames the detection of directionalconduction is marginal because of leakage in both directions andamplification does not fully solve the problem in that it is susceptibleto a.c. pickup particularly when the amplifier is connected to the hotside of the line.

The above noted problems are avoided to some extent in the systemdisclosed in U.S. Pat. No. 3,441,356. ln that system a single polaritysupply is utilized to produce the flame responsive current and therelative conduction from a positive electrode is compared with that froma negative electrode to establish the presence of flame. However, inthat type of system an inadvertent short circuit to the flame sensingelectrode will produce a d.c. current that cannot be distinguished froma flame supported signal. Other common problems of this as well as otherburner control systems are associated with the electronic elements usedto monitor the signals produced by the flame rectification current.Typically, an electronic switching element such as a silicon controlledrectifier is gated by the flame signal to produce a desired controlsignal. False triggering of such conductor devices by transients isrelatively common and reduces overall system reliability.

The object of this invention therefore, is to provide an improved flameresponsive control system for fuel burners that is both reliable and ofreasonable cost.

SUMMARY OF THE INVENTION The invention is characterized by the provisionof a flame monitoring circuit in which a storage capacitor is connectedbetween the hot line of an a.c. power supply and a flame electrodedisposed so as to be bathed in the flame being monitored. The storagecapacitor is electrical current isolated from the neutral line of thepower source so as to pass only that current circulating between the hotline and the grounded burner providing the flame being monitored.Because of its rectification properties, the flame causes a flow ofdirect current that charges the storage capacitor providing a flameindicating signal voltage. A control circuit powered by the a.c. sourceis coupled to the storage capacitor so as to respond to either thepresence or absence thereon of a d.c. signal voltage with respect to thehot line. By isolating the flame rectified current ina ground loop andutilizing the resultant d.c. signal voltage with respect to the hotline, the signal to noise ratio of the system is greatly enhancedwithout extensive filtering and the requirement for an isolationtransformer is eliminated.

In a featured embodiment of the invention above described, the controlcircuit includes a silicon-controlled rectifier (SCR) that is gated bythe flame indicating signal voltage to supply power to suitable load.Generally the load consists of an electrical operator for controlling avalve that supplies fuel to the burner being monitored. The load canalso include a pulse transformer for providing ignition pulses toelectrodes disposed so as to ignite fuel emanating from the burner. Inthat case, one of the spark electrodes is preferably utilized tofunction also as the flame sensing electrode that carries the flamerectified current.

Another feature of the invention constitutes a sampling circuit-forperiodically sampling the energy level stored in the storage capacitor.The sampling circuit includes a discharge capacitor coupled to thestorage capacitor so as to receive charging current therefrom and acomplementary silicon-controlled rectifier periodically activated todump the energy from the discharge capacitor into the gate circuit ofthe silicon controlled rectifier. Preferably, the complementarysiliconcontrolled rectifier is fired at zero-crossings of the a.c. powersource immediately preceding those half cycles during which flamerectified current is produced. This insures that the signal level attime of discharge is dependent only upon flame rectified current flowand not upon any temporary charge produced by alternating current flowthrough the high impedance path provided by the flame. Also, thepossibility of inadvertent firings ofthe silicon-controlled rectifier bystray signals is substantially reduced.

DESCRIPTION OF THE DRAWINGS These and other objects and features of theinvention will become more apparent upon a perusal of the followingdescription taken in conjunction with the accompanying drawings therein:

FIG. l is a schematic circuit diagram showing a preferred embodiment ofthe invention;

FIGS. 2a, 2b, 2C are graphs showing various waveforms present in thecircuit of FIG. l; and

FIG. 3 is a schematic circuit diagram of another preferred embodiment ofthe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 thereis shown a circuit 11 for monitoring the presence or absence of flame ina region 12 directly adjacent a fuel burner 13. The monitoring network1l is retained by an electrically conductive housing 14 and includes asensing circuit 15 connected between a control circuit 16 and a flameelectrode 17 disposed in the flame region l2. Power is supplied to thenetwork 11 by connection of` first terminal 18 and a second terminal 19to a conventional a.c. source. The terminal 18 is connected to a line 20while a third terminal 21 is connected to the conductive housing 14.Fuel such as natural gas or oil, for example, is supplied to the burner13 through a supply pipe 22 and a solenoid controlled valve 23.

The sensing circuit 15 includes a storage capacitor C1 and Resistor R1connected in series between the terminal 18 and the flame electrode 17.Also included in the sensing circuit 15 is a series combination of afilter resistor R2, a discharge capacitor C2 and a signal resistor R3connected across the storage capacitor C1. A further element of thesensing circuit 15 is a sampling circuit comprising a complementarysilicon-controlled rectifier (CSCR) having an anode connected to theline 20 and a cathode connected to a junction between the resistor R2and the capacitor C2. The gate electrode of the CSCR is connected to theline 20 by a diode D1 and to the terminal 19 by a resistor R4.

Included in the control circuit 16 are a parallel combination of asolenoid relay winding 3l and a filter capacitor C3 connected to theline 20 by a capacitor C4 and to the terminal 19 by a diode D2.Additional elements of the control circuit 16 are an ignitiontransformer Tl and a silicon controlled rectifier (SCRl). The primarywinding 32 of the transformer T1 is connected between the capacitor C4and the anode of the SCR, the cathode of which is connected to the line20. Signal, coupling between the sensing circuit 15 and the controlcircuit 16 is provided by a connection between the gate of the SCRI andthejunction between the discharge capacitor C2 and the signal resistorR3. The secondary winding of the transformer Tl is connected between aspark electrode 34 positioned in the flame region l2 and the flameelectrode 17 which also serves as a second spark electrode.

A start-up circuit 4l is also included in the network l1 shown in FIG.l. The start-up circuit 41 includes a parallel combination ofa filtercapacitor C and a relay winding 42 connected in series with a resistorR5 and a diode D3 across the input terminals 18 and 19. Actuated by thewinding 42 is a switch with a movable contact 43 connected to the inputterminal 19. With the winding 42 deenergized, the movable contact 43engages a stationary contact 44 connected to the junction betweencapacitor C1 and resistor R1 by a resistor R6 and a diode D4.Energization of the winding 42 moves the contact 43 into engagement witha stationary contact 45 connected to one end of a solenoid 37 thatcontrols the fue] supply valve 23. The other end of the solenoid 37 isconnected to a normally open switch 36 that is activated by the winding31.

During installation, the network l1 is connected to a conventional l l5volt power source with terminal 18 connected to the hot line andterminal 19 connected to the neutral wire. Assuminga typical three wiresupply, the terminal 2l is connected to the ground wire assuring thatthe housing 14 is at the ground potential of the supply pipe 22 and theburner 13.

To initiate operation of the burner 13, an on-offswitch 47 is closed soas to produce on line 20 the sine wave voltage illustrated in FIG. 2a.Because of a time delay provided by the capacitor C5 the winding 42 isnot immediately energized and the contacts 43 and 44 remain engaged toprovide a current path through the storage capacitor Cl, the resistor R6and the diode D4 during positive half cycles on the line 20.Consequently, there is built up in the storage capacitor C1 a chargehaving the polarity indicated in FIG. 1. This charge on storagecapacitor C1 which is, for example, in the range of about l0 volts isillustrated by the waveform in FIG. 2b shown in time alignment with thewaveform in FIG. 2a. The diode Dl biases the gate of the CSCR negativewhen the voltage on line 20 is positive causing it to conduct and biasesthe gate positive by the amount of forward drop in the diode D1 whilethe voltage on line 20 is negative insuring non-conduction of the CSCR.Thus, as the CSCR becomes conductive during a portion of each positivehalf cycle on line 20 so as to shunt the capacitor C2 through resistorR3 and the capacitor C1 through resistor R2. However, during negativehalf cycles on line 20, the CSCR is nonconductive and dischargecapacitor C2 is charged through R2 in accordance with the chargeremaining on storage capacitor Cl. As the voltage on line 20 goespositive to fire the CSCR, any appreciable charge on discharge capacitorC2 appears as a positive pulse across the resistor R3 to fire the SCR 1.

Triggering of the SCR 1 allows the capacitor C4 which on the previoushalf cycle was charged with the polarity indicated in FIG. l todischarge through the primary winding 32 of the transformer T1. Thisproduces a high voltage pulse in the secondary 33 and a resultant sparkbetween the electrodes 17 and 34. During the next negative half cycle online 20 the capacitor C4 is again charged by current flow through thediode D2 and the relay winding 3l. This operation continues producingduring each cycle on line 20 an ignition spark in the region 12 betweenelectrodes 17 and 34 and a surge of current through the relay winding 31maintaining energization thereof and resultant closure of the switch 36.As described above, the winding 42 is not immediately energized becauseof the time constant exhibited by the capacitor C5. After a certaindelay, however, the winding becomes energized moving the contact 42 intoengagement with the fixed contact 4S. Simultaneously, contacts 43 and 44are opened to terminate the supply of line current to the storagecapacitor C1 which, however, retains sufficient charge to continuefiring the SCR 1 for a given ignition period in the manner describedabove. During that period both switch 36 and contacts 44, 45 are closedto energize the valve solenoid 37. In response to energization tosolenoid 37, the valve 23 opens initiating fuel flow to the burner 13.

Assuming that the fuel fed to the burner 13 is ignited, a flame appearsin the region 12 occupied by the electrodes 17 and 34. As is well known,that flame acts as an imperfect diode that may be represented (as showndotted in FIG. 1) by a perfect diode with a high resistance in seriesand another high resistance in parallel with the combination. The flameproduced diode is polarized such that the greater current flow occursthrough the flame on the positive half cycles illustrated in FIG. 2athus maintaining on storage capacitor Cl the charge illustrated in FIG.2b. Thus, the maintenance of a charge on the storage capacitor C1 isindicative of current flow supported by flame in the region 12. Thepresence of charge on the capacitor C1 insures continued operation ofthe tensing circuit 15 and control circuit 16 in the manner describedabove to insure continued flow of fuel through the valve 23.

If at any time the flame in region 12 is extinguished, the network 1ltries for re-ignition during a brief ignition period. As describedabove, this period is provided by the storage capabilities of thecapacitor C1 which continues to supply current to the dischargecapacitor C2 for a limited period even the absence of continuing currentflame rectified currentflow. However, if the flame is not re-ignitedwithin the short ignition period, the absence of flame rectified currentflow will result in discharge of the capacitor C1 and eliminateperiodiccharging current flow to the discharge capacitor C2. Consequently, nofurther pulses will be produced acrossv the resistor R3 to fire the SCR1, which will remain non-conductive terminating periodic discharge ofthe capacitor C4. This in turn will eliminate energizing current flowthrough the winding 3l to open the switch 36 and de-energize solenoid37.

Thus, a prolonged loss of flame in the region 12 automatically resultsin closing of valve 23 to prevent further fuel flow to the burner 13.Furthermore, because the winding 42 in the start-up circuit 41 remainsenergized to prevent engagement of contacts 43 and 44 and, accordingly,charging current flow through the storage capacitor C1, a new try forignition can be initiated only by a loss of power between the terminals18 and 19. That occurence caused for example by opening the switch 47,will de-energize winding 42 allowing Contact to befmade between contacts43 and k44 and producing another try for ignition in the mannerdescribed above. It will be appreciated that this reignition procedure,as described above, will be required also in the event that ignition isnot initially achieved within the ignition period provided by retainedcharge in the storage capacitor C1.

It will be noted with regard to the network 11 shown in FIG. 1, that thesensing circuit l5 is current isolated from the control circuit 16. Anycurrent available for charging the storage capacitor C1 and accordinglythe discharge capacitor C2 must be supported by flame in the region l2which completes a path to the ground circuit including the burner 13 andthe ground terminal 21. Thus, any energy available in the dischargecapacitor C2 for producing a flame signal across the resistor R3 that inturn triggers and trigger SCR 1 can result only from current flowthrough a flame in region 12. Furthermore, by utilizing as a flamesignal a pulse of stored energy having a level dependent upon the flamecondition being sensed, no signal amplification is required. For thesereasons highly reliable signal infomiation is provided and, in addition,circuit isolating the neutral terminal 19 from the ground circuiteliminates the`need for isolation transformers. y Of further note is theutilization of the flame signal applied to the SCR 1 inthe controlcircuit 16 with respect to the hot line 20 thereby substantiallyreducing the effect on the sensing signal of a.c. present within thesystem. Y

It will be appreciated however, that on an instantaneous basis some a.c.effects are present in the measuring circuit 15. As noted above theflame acts as an imperfect rather than a perfect diode and does supporta small component `of a.c. This is demonstrated in FIG. 2b wherein thenegative flame voltage on capacitor C1 scribed sampling of the chargeenergy stored in the discharge capacitor C2 at a particular time duringthe a.c. cycle. The sampling diode D1 triggers the CSCR to discharge thecapacitor C2 at each positive going transition of the voltage on line20. Those are the particular a.c. zero crossings which initiate a.c.current flow -in the same sense as the d.c. flame current through thestorage capacitor C1. As illustrated FIG. 2b, it is at those particulartimes that the opposite half cycle a.c. effects on the capacitor C1 havecompensated each other leaving a steady state d.c. signal responsiveonly to the d.c. current provided by the sensed flame condition.

Referring now to FIG. 3, there is illustrated another embodiment 51 inwhich components identical to those shown in FIG. l are givencorresponding reference numerals. A sensing circuit 52 is identical tothe sensing circuit 15 of FIG. 1 except that a primary winding 53 of atransformer T2 replaces the resistor R3. A control circuit 54 includes asilicon-controlled rectifier SCR2 and a load resistor RL connected inseries across input terminals 18 and 19. Also included in the controlcircuit 54 is a secondary winding 55 of the transformer T2 connectedbetween the gate of the SCR2 and the junction between the resistor RLand the cathode of the SCR2.

The operation of the embodiment 51 is similar to that described abovefor embodiment l1. However, in this case the presence of flame in region12 is indicated at each discharge of the discharge capacitor C2 by apulse V through the primary winding 53. The resultant pulse in thesecondary winding 55 fires the SCR2 so as to provide energizing currentfor the load RL. It should be noted that SCR2 is poled opposite to thatshown in embodiment 11 and may be powered directly from the lineincreases during the positive half cycle on signal line 20 6 and thendecreases to its steady state value at the con.

instead of from a previously charged capacitor. It will be obvious thatthe load RL could include, for example, a valve controlling relay or anignition transformer as in embodiment 11. Also, the starter circuit 4lshown in FIG. 1 could be similarly employed in embodiment 51.

Obviously, many modifications and variations of the present inventionare possible in light ofthe above teachings. It is therefore, to beunderstood that within the scope of the appended claims the inventioncan be practised otherwise than as specifically described.

What is claimed is:

1. Circuit apparatus for controlling a fuel burner and comprising asource of alternating current voltage;

a flame sensing electrode for disposition adjacent a fuel burner in theregion occupied by flame emanating therefrom;

direct current sensing means coupled to said flame sensing electrode andadapted to provide a flame signal in response to current rectified byflame in said region;

fuel control circuit means for controlling the flow of fuel to said fuelburner; said fuel control circuit means being coupled to said directcurrent sensing means and adapted to provide fuel flow to said burner inresponse to said flame signal;

direct current source means for supplying direct current to said directcurrent sensing means; and

operator means operative in one state to simultaneously establish flowof direct current from said direct current source to said direct currentsensing means and prevent said fuel control circuit means from providingfuel flow to said burner, and operative in a second state tosimultaneously prevent flow of direct current from said direct currentsource to said direct current sensing means and allow said fuel controlcircuit means to provide fuel flow to said burner in response to saidflame signal.

2. Circuit apparatus according to clairn 1 wherein said direct currentsensing means comprises electrical energy storage means for storingenergy present in said current rectified by flame, and said flame signalis provided by a given stored energy level in said energy storage means.y

3. Circuit apparatus according to claim 2 wherein said energy storagemeans comprises a capacitor that is charged by said current rectified byflame and by said direct current supplied by said direct current sourcemeans.

4. Circuit apparatus according to claim 2 wherein said operator meanscomprises switch means operative in said one state to connect saiddirect current energy source to said energy storage means, and operativein said second state to disconnect said direct current energy sourcefrom said energy storage means.

5. Circuit apparatus according to claim 2 wherein said direct currentsource comprises rectifier means connected to said source of alternatingcurrent voltage.

6. Circuit apparatus according to claim 2 wherein said fuel controlcircuit means comprises an electrically controlled actuator for a valvesupplying fuel to said burner.

7. Circuit apparatus according to claim 6 wherein said operator meanscomprises switch means operative in said one state to simultaneouslyconnect said direct current energy source to said energy storage meansand to prevent energization of said valve actuator, and in said secondstate to simultaneously disconnect said direct current energy sourcefrom said energy storage means and allow energization of said valveactuator.

8. Circuit apparatus according to claim 7 wherein said switch meanscomprises double-throw switch means that in one position connects saidenergy storage means to said direct current energy source anddisconnects said valve actuator from any source of energization, and ina second position disconnects said energy storage means from said directcurrent energy source and connects said valve actuator to said fuelcontrol circuit means.

9. Circuit apparatus according to claim 8 wherein said valve actuatorcomprises an operating solenoid and an SCR that provides energizingcurrent to said operating solenoid in response to said flame signal.

10. A burner safety timer control circuit adapted to be connected to asource of alternating current voltage and to rectification flame sensormeans of a fuel burner, including: capacitive means and rectificationmeans connected to said alternatingcurrent voltage source uponenergization of said burner control circuit to supply an initial chargeto said capacitive means; current drain means connected to saidcapacitive means to slowly drain said capacitive means of said initialcharge; solid-state current control means having an input connected tosaid capacitive means and output means adapted to be connected tooperate said fuel burner when said capacitive means charge ismaintained; andV connection means adapted to connect said rectificationflame sensor means to said capacitive means to supply a charge to saidcapacitive means when said fuel burner has a proper flame to maintainsaid charge and keep said solid-state current control means output meansoperating said fuel burner.

l1. A circuit according to claim l0 including isolation means forisolating said capacitive means from the supply of charge from saidrectification means.

12. A circuit according to claim ll including delay means for operatingsaid isolation means a given period after energization of said burnercontrol circuit.

13. A circuit according to claim 12 wherein said isolation meanscomprises a relay means.

1. Circuit apparatus for controlling a fuel burner and comprising asource of alternating current voltage; a flame sensing electrode fordisposition adjacent a fuel burner in the region occupied by flameemanating therefrom; direct current sensing means coupled to said flamesensing electrode and adapted to provide a flame signal in response tocurrent rectified by flame in said region; fuel control circuit meansfor controlling the flow of fuel to said fuel burner; said fuel controlcircuit means being coupled to said direct current sensing means andadapted to provide fuel flow to said burner in response to said flamesignal; direct current source means for supplying direct current to saiddirect current sensing means; and operator means operative in one stateto simultaneously establish flow of direct current from said directcurrent source to said direct current sensing means and prevent saidfuel control circuit means from providing fuel flow to said burner, andoperative in a second state to simultaneously prevent flow of directcurrent from said direct current source to said direct current sensingmeans and allow said fuel control circuit means to provide fuel flow tosaid burner in response to said flame signal.
 2. Circuit apparatusaccording to claim 1 wherein said direct current sensing means compriseselectrical energy storage means for storing energy present in saidcurrent rectified by flame, and said flame signal is provided by a givenstored energy level in said energy storage means.
 3. Circuit apparatusaccording to claim 2 wherein said energy storage means comprises acapacitor that is charged by said current rectified by flame and by saiddirect current supplied by said direct current source means.
 4. Circuitapparatus according to claim 2 wherein said operator means comprisesswitch means operative in said one state to connect said direct currentenergy source to said energy storage means, and operative in said secondstate to disconnect said direct current energy source from said energystorage means.
 5. Circuit apparatus according to claim 2 wherein saiddirect current source comprises rectifier means connected to said sourceof alternating current voltage.
 6. Circuit apparatus according to claim2 wherein said fuel control circuit means comprises an electricallycontrolled actuator for a valve supplying fuel to said burner. 7.Circuit apparatus according to claim 6 wherein said operator meanscomprises switch means operative in said one state to simultaneouslyconnect said direct current energy source to said energy storage meansand to prevent energization of said valve actuator, and in said secondstate to simultaneously disconnect said direct current energy sourcefrom said energy storage means and allow energization of said valveactuator.
 8. Circuit apparatus according to claim 7 wherein said switchmeans comprises double-throw switch means that in one position connectssaid energy storage means to said direct current energy source anddisconnects said valve actuator from any source of energization, and ina second position disconnects said energy storage means from said directcurrent energy source and connects said valve actuator to said fuelcontrol circuit means.
 9. Circuit apparatus according to claim 8 whereinsaid valve actuator comprises an operating solenoid and an SCR thatprovides energizing current to said operating solenoid in response tosaid flame signal.
 10. A burner safety timer control circuit adapted tobe connected to a source of alternating current voltage and torectification flame sensor means of a fuel burner, including: capacitivemeans and rectification means connected to said alternating currentvoltage source upon energization of said burner control circuit tosupply an initial charge to said capacitive means; current drain meansconnected to said capacitive means to slowly drain said capacitive meansof said initial charge; solid-state current control means having aninput connected to said capacitive means and output means adapted to beconnected to operate said fuel burner when said capacitive means chargeis maintained; and connection means adapted to connect saidrectification flame sensor means to said capacitive means to supply acharge to said capacitive means when said fuel burner has a proper flameto maintain said charge and keep said solid-state current control meansoutput means operating said fuel burner.
 11. A circuit according toclaim 10 including isolation means for isolating said capacitive meansfrom the supply of charge from said rectification means.
 12. A circuitaccording to claim 11 including delay means for operating said isolationmeans a given period after energization of said burner control circuit.13. A circuit according to claim 12 wherein said isolation meanscomprises a relay means.